1. Field of the Invention
The invention relates to a method of recording information concerning a device operating fault. The invention is particularly applicable in diagnosing a failure of a device in a motor vehicle (for example a drive train of the vehicle), in order to facilitate the repair of the failed device.
2. Description of the Related Art
Modern motor vehicles use on-board computers to control numerous electrical or mechanical devices, such as, for example, to control an injection system of the engine, to control an automatic or robotized gearbox, and so on.
To guarantee both safety of the user regarding a possible failure of a device or of a component of a control system and the repair of the failed device by a repairer, these computers also incorporate means for implementing various electrical or functional diagnostic methods.
When a failure of a device is diagnosed for the first time, the computer records in a nonvolatile memory information relating to the observed fault; this information notably comprises a code associated with the observed fault, and possibly values of parameters characteristic of the operation of the device, also called context data. The context data can be generic, that is, identical for all the observed faults, or even specific to a particular fault; they can in this case be different for different faults. For example, in the case of a fault associated with a speed control, the speed set point and the value of the speed measured on detection of the fault are recorded. In all cases, the recorded information is used to help the subsequent repairer by providing him with additional information on the conditions in which the fault was detected.
In some current recording methods, the context data is stored only on the first occurrence of the fault. It is therefore impossible to know whether the stored conditions truly correspond to particular conditions where the fault regularly occurs, or simply to the “any” conditions prevailing on the chance occurrence of the fault. As an example, for an electrical fault where the engine speed stored on the first occurrence of the fault is 2500 rpm, there is nothing to tell the repairer whether it is an ongoing fault occurring for the first time at the speed of 2500 rpm, or an intermittent fault occurring only at a speed approaching 2500 rpm, and which would be due, for example, to a resonance effect.
This solution is not satisfactory, because the information stored is often insufficient to allow a rapid repair, particularly in the case of intermittent faults (for example electrical faults due to bad contacts) and/or in the case of complex faults (for example operating faults occurring in precise and restricted conditions).
In other current recording methods, the context data is recorded on each occurrence of the fault that it characterizes, which facilitates the diagnostic procedure.
This solution is, however, extremely costly in terms of nonvolatile memory. In practice, if the context data stored on detection of a fault represents a size of n bytes, the occurrence m times of this fault will therefore lead to the use of n*m bytes of memory. The richness of the transmitted information is therefore rapidly limited by the size of the memory.